Hi Kenneth
The SOAP API doesn't expose the temporal ID that is used in the GUI. To access this you will have to use the REST API. Here you can find the property "draftInvoiceNumber" on /invoices-experimental/drafts/
If you need to link the SOAP API CurrentInvoice handle Id with the REST endpoint you can take a look at the "soap" object on the draft:
"soap": {
"currentInvoiceHandle": {
"id": 15
}
Or the other way around: use "soap.currentInvoiceHandle.id" to filter for the SOAP Id you're looking for (15 in this case):
https://restapi.e-conomic.com/invoices-experimental/drafts?filter=soap.currentInvoiceHandle.id$eq:15